This invention relates to silicone rubber compositions and methods for molding silicone rubbers for use in the extrusion molding of tube, sheet, wire and cable coatings, and to a method for molding the corresponding silicone rubbers. More particularly, this invention relates to silicone rubber compositions with a new combination of organoperoxide curing agents.
Silicone rubbers are frequently used for the extrusion molding of tube, tape, sheet, wire and cable coatings, and the like because these rubbers offer an excellent heat resistance, cold resistance, weathering resistance, and electrical characteristics. 2,4-Dichlorobenzoyl peroxide has been used as a curing agent for effecting the cure of the silicone rubber compositions used as the feedstock in these extrusion molding applications. However, various problems are associated with silicone rubber compositions that contain this organoperoxide: they evolve an unpleasant odor when cured, their cure yields moldings that have a sticky, or tacky surface, and blooming occurs onto the surface of their silicone rubber moldings. This blooming is caused by the gradual bleed out of curing agent decomposition products with an associated whitening of the molding's surface. These decomposition residues can also cause environmental pollution. A number of investigations have already been carried out in order to solve these problems. For example, Yonezawa et al. in U.S. Pat. No. 4,260,536 propose a method for preparing low-surface-tack silicone rubber moldings that uses bis(2,4-dichlorobenzoyl)peroxide and di-tert-butyl peroxide as curing agent. However, this is not a completely satisfactory method because it still leads to the evolution of an unpleasant odor during cure and because it does not completely suppress surface tack in the molding. Japanese Patent Application Laid Open (Kokai or Unexamined) Number Sho 59-18758 (18,758/1984) teaches a method that uses bis(ortho-methylbenzoyl)peroxide as curing agent for silicone rubber compositions. This method avoids the evolution of unpleasant odor and does not cause blooming. Dorn, et al. in U.S. Pat. No. 4,743,671 teach a method that uses bis(para-methylbenzoyl)peroxide as curing agent for silicone rubber compositions, and this method also solves the same problems. However, the silicone rubber moldings afforded by thermosetting a silicone rubber composition prepared by the former method are susceptible to yellowing, or taking on a yellowish tinge. Silicone rubber compositions prepared by the latter method suffer from slow cure rates and tend during curing to foam with the formation of extremely small bubbles, creating voids in the rubber.
An object of the present invention is to provide a silicone rubber composition that does not foam or evolve an unpleasant odor during its cure and that cures into low-surface-tack, non-yellowing silicone rubber moldings. An additional object of the invention is to provide a method for molding silicone rubbers.